Most people probably thinks very little about the platypus – if it is at all.
But this strange Australian mammal, with its webbed feet and trembling tail, is actually a treasure trove of genetic information.
Platypus (Ornithorhynchus anatinus) is part of a unique mammalian group known as monotreme. This group also contains echidnas (think Knuckles of Sonic the Hedgehog but really). They are the only group of mammals that lay eggs. As such, monotreme is the best candidate for the study of mammalian evolution.
Research published in the journal on Wednesday Earth provides, for the first time, a complete overview of the genus of the platypus, which reveals new information about the evolution of this strange group of mammals – and insight into the origin of human DNA.
Essential background – The three main groups of mammals are: monotreme, euthanasia and marsupials.
The largest group are by far euthanasia, which includes animals that feed their young in the mother’s placenta (hence the infraclass is sometimes referred to as “placenta”). People fit into this category.
Marsupials and euthanasia are often grouped together in a subclass known as therian mammals, but monotremically differ from other mammals in that they deserve their own separate category.
Monotreme contains only two animals that survive today: the platypus and the echidna.
Platypus are semi-aquatic creatures and echidna are land dwellers. Both have a common habit that is unique to monotreme: egg laying.
What has been discovered – For the first time, the new research offers a complete chromosome map of the platypus genome, together with a less complete map of the echidna genome.
The scientists used genetic data to analyze everything from the platypus’ eating habits to its swimming routine. The scientists found, for example, that the water-based platypus possesses much less “odorous” or odorous genes compared to the terrestrial echidna.
These genetic data confirm the semi-aquatic lifestyle of the platypus, which closes its nasal cavity and eyes in the water and instead relies on other senses – such as electrical stimuli – to detect prey.
But given the unique reproduction strategy of the monotreme, the researchers focus specifically on its unique sex chromosomes. The monotreme has 10 sex chromosomes: five X chromosomes and five Y chromosomes. Humans, meanwhile, have two sex chromosomes.
Through their analysis, researchers found similarities between platypus and birds, but not so much between platypus and humans.
Guojie Zhang, the corresponding author on the study and a professor in the biology department at the University of Copenhagen, tells Conversely, “From the genome sequence, we found that these ten sex chromosomes have no homologue to the X / Y chromosomes in humans, but are more similar to the ZW chromosome of birds in the genomic succession structure.”
The complex sex chromosomes in monotreme exhibit unusual interactions during and after meiosis, which is a type of cell division.
“During the meiosis process, the homologous regions of the pair of chromosomes may correspond to each other. Therefore, the sex chromosomes of platypus may form a ring structure. during meiosis process, “says Zhang.
The ring structure is perhaps the most interesting finding of the study. This type of chromosome ring has been found in plants, but never before in animals, which makes it a groundbreaking finding.
The study also compared platypus genes with other animal genes – ranging from Tasmanian demons to humans – to form an image of the chromosome of ancient mammals, also known as a karyotype.
Using these species, they were able to understand how platypus genes differ from their ancient mammalian ancestors, and how modern platypus possess five different X chromosomes. They were also able to trace the last existing common ancestor of humans and platypus 163 to 191 million years ago.
But what’s even cooler: by examining the chromosomes of the platypus, scientists were able to better understand the origin of our own human DNA.
The study states that the scientists ‘confirmed that the X chromosome in humans was derived from the fusion of an original therian X chromosome with an autosomal region after the abnormality of marsupials.’
Why it matters – Without a complete map of the chromosomes of an animal, it is difficult to fully comprehend its evolution. That is why a genomic analysis of this tiny, tiny concept of mammals is so valuable to the scientific community.
According to the study, the map produced for the platypus and echidna enables us to deduce the genomic changes that occurred in the ancestral monotreme and other mammals.
In other words, by looking at the DNA of platypus, we can learn more about the genetic changes that took place in ancient mammals, to help us better understand the evolution of living animals today.
For example, the study found that “gene families associated with the immune response and hair growth were significantly expanded in the ancestor of mammals, possibly contributing to the evolution of immune adaptation and fur in mammals, respectively.”
The research especially expands our understanding of the reproductive processes of mammals. Monotreme serves as an interesting transition point between oviparous (egg-laying) reptiles and live animals – like humans – which grow an embryo in the body.
The study reads:
“Monotremes provide the key to understanding how viviparity has evolved in mammals.”
The study found, for example, that the platypus contains fewer copies of an egg-producing protein, called vitellogenin, than reptiles, which means they are not as dependent on these proteins to lay eggs. But the existence of this gene can explain why platypus lays eggs in the first place.
“For example, birds have three copies of the vitellogenin gene. While we are in other mammals there (like humans), we have no vitellogenin gene,” says Zhang. ‘But the monotreme still has one copy of the vitellogenin gene [that] maintain the same function in birds. This may explain why they can still produce egg. ‘
“Our findings will be of great interest to people who want to investigate the reproductive systems in monotreme and also the organization of the sex chromosome during the meiosis process,” Zhang adds.
What’s next – As the researchers acknowledge, there is still much we do not yet know about these egg-laying mammals.
“Unfortunately, it remains a mystery why they have so many sex chromosomes,” says Zhang.
But by providing such a clear genetic makeup of monotreme, this study takes a new and exciting step forward in the world of mammalian research.
“With the complete high-quality chromosome-scale genome, we can allow ourselves to investigate the functions of the non-coding genomic sequence that are important for the evolution of all mammals,” says Zhang.
Summary: Egg-laying mammals (monotreme) are the only mammalian subgroup for therians (marsupials and euthanasia) and provide important insights into evolution of mammals. Here we generate and analyze reference genomes of the platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus), representing the only two existing monotreme lineages. The almost complete composition of the platypus genome anchored almost the entire genome to chromosomes, which significantly improved genome continuity and gene recording. Together with our echidna sequence, the genomes of the two species enable us to detect the ancestral and sex-specific genomic changes that form monotreme and mammalian evolution. We provide evidence that the monotreme sex chromosome complex is derived from an ancestral chromosome ring configuration. The formation of such a unique chromosome complex could possibly be facilitated by the extraordinarily extensive interactions between the multi-X and multi-Y chromosomes shared by the autosomal homologues in humans. Further comparative genomic analyzes unravel clear differences between monotreme and therians in haptoglobin genes, lactation genes, and chemosensory receptor genes for odor and taste underlying the ecological adaptation of monotreme.